1. Field of the Invention
Embodiments of the invention generally relate to plasma processing in the field of semiconductor devices. More specifically, the present invention relates to methods for igniting a plasma in a process chamber during the fabrication of semiconductor devices.
2. Description of the Related Art
In some semiconductor fabrication processes, a plasma of positive ions, negative ions and neutrals is introduced to a process chamber to assist in semiconductor device formation. During processing, the plasma is repeatedly extinguished and re-ignited as new substrates are introduced to the chamber for processing. For some processes, the plasma may be extinguished and re-ignited between steps of a multi-step process being performed on a substrate within a single chamber. In addition, the plasma may self-extinguish, or drop-out, unexpectedly in the midst of a process step if the process conditions destabilize, thereby necessitating re-ignition of the plasma. At times, re-ignition of the plasma may be difficult if harsh conditions (i.e., conditions unfavorable to plasma ignition) exist in the process chamber.
For DC plasma ignition, the “Paschen curve” is generally followed, which states that the voltage required to ignite a plasma is a function of gas pressure and source-substrate dimension. Therefore, it is normally assumed that the greater the cathode voltage potential available, the higher the probability of plasma ignition. For this reason, the conventional wisdom is generally to apply and hold full cathode ignition voltage until a plasma ignites. This approach generally works if the anode (generally, the substrate) is held at a lower potential and the potential difference is sufficiently large.
However, in chamber designs were the anode (substrate) is electrically isolated, the anode can float towards the cathode potential. As the anode floats towards the cathode potential, the potential difference between the cathode and the anode is reduced. This reduction in potential difference may be sufficient to prevent plasma ignition. Moreover, the longer the cathode voltage is held at full ignition without a plasma ignition, the closer the floating anode potential will approach to the cathode potential, thereby further deterring plasma ignition.
Thus, there is a need in the art for an improved method for igniting (or re-igniting) a plasma in a semiconductor process chamber.